1. Field of the Invention
This invention relates to an improved process for preparing aromatic dicarboxylic acid chlorides, in high purity in solution or slurry form, by reacting aromatic dicarboxylic acids with phosgene in the presence of a weak tertiary amine base and an inert chlorinated paraffinic hydrocarbon solvent, for direct use as acylating agents.
2. Brief Description of the Prior Art
Aromatic dicarboxylic acid chlorides are well known as acylating agents, particularly, for bisphenols, glycols and diamines in forming aromatic polyesters and polyamides. Optimum conditions for the polymerization usually include the use of aromatic acid chlorides in solution or slurry form for achieving high molecular weight and controlled molecular weight distributions.
Processes for the preparation of carboxylic acid chlorides from the reaction of carboxylic acids with phosgene are known in the art, and are exemplified in the following references.
U.S. Pat. No. 3,869,485 (1975) describes an improved process for the preparation of monocarboxylic acid halides by reaction of monocarboxylic acids or anhydrides with phosgene in the presence of a benzimidazole or benzotriazole catalyst. However, the process requires reaction temperatures above 50.degree. C., preferably from about 100.degree. to 180.degree. C. at atmospheric pressure, during which hydrogen chloride gas is evolved. The high temperatures required, above ambient temperature, adds to the cost of the process and evolution of hydrogen chloride gas at these temperatures may present corrosion problems to the apparatus employed.
British Pat. No. 1,415,980 (1975) describes a process for preparing acid chlorides by reacting carboxylic acids with phosgene at an elevated temperature in the presence of a tertiary amine catalyst. However, the reaction requires elevated temperatures, preferably of about 120.degree. to 160.degree. C., and since an inert organic solvent is not employed, does not result in a solution or slurry of the acid chloride suitable as an acylating agent.
U.S. Pat. No. 3,318,950 (1967) describes a process for preparing carboxylic acid chlorides by reacting intermolecular anhydrides of carboxylic acids with phosgene in the presence of a carboxamide, optionally in the presence of an inert organic solvent. However, the reaction has the disadvantage in that it requires the use of acid anhydrides rather than the acids themselves for the direct production of acid chlorides.
French Pat. No. 732,078 (1932) describes a process for preparing organic acid halides by reacting a carbonyl halide such as phosgene, and an organic acid, in a temperature range including 10.degree.-50.degree. C., and in the presence of an inert organic solvent including chlorinated paraffinic hydrocarbons, using for each acid group of the organic acid, i.e., carboxylic or sulfonic acid group, at most an equivalent amount of a nitrogenous base or salt thereof.
We have unexpectedly found that in preparing aromatic dicarboxylic acid chlorides, in solution or slurry form, from the corresponding dicarboxylic acid by reaction with phosgene at ambient temperatures from about 10.degree. to 50.degree. C., in an inert chlorinated paraffinic hydrocarbon solvent, at least 1.02 equivalents of a weak tertiary amine base per equivalent of carboxylic acid group must be present in the reaction mixture, throughout the reaction period, to obtain the acid chloride in at least about 90% yield of theory, based on the total dicarboxylic acid employed in the reaction. In addition to obtaining high yields of diacid chloride, the process has the advantages of not requiring acid anhydrides as starting materials and allows the production of diacid chlorides from aromatic dicarboxylic acids at ambient temperatures, e.g. 10.degree.-50.degree. C., thus reducing the cost of the process and inhibiting potential corrosion and pollution problems associated with the use of higher temperatures as in the prior art.